Envelope for transparencies

ABSTRACT

An improved envelope for transparencies for overhead projectors comprising a substantially rectangular pocket made of a transparent sheet material, the pocket having two longitudinal side edges connected by shorter side edges, and being open at least along one side edge for insertion of a transparency, at least one opaque flap attached in a foldable manner along one of the side edges by means of a flexible adhesive tape, the tape comprising a flexible polyolefin backing having a thickness of from about 30 micrometers or less, and coated on one surface of the backing is an adhesive layer, wherein the tape has a flexibility measurement of about 2.5 grams, and the flap is capable of remaining in the unfolded position on an overhead projector stage with minimal adhesive transfer at the operating temperature of the overhead projector.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The invention relates to an envelope for holding transparencies used in overhead projectors. Specifically, the envelopes include at least one opaque flap attached to the envelope using an adhesive tape having increased flexibility.

2. Description of the Related Art

Business presentations, lectures and the like often are augmented by the use of imaged transparencies, which are exhibited to the viewers on a screen by means of an overhead projector (OHP). The individual transparencies are delicate, and easily damaged in storage or transit. Various types of covers, folders and envelopes have been provided in order to enable professional presentations, as well as interim protection of the transparencies.

U.S. Pat. No. 3,875,693 discloses the framing of a transparency in a frame made from carton, paperboard and the like. The frame is meant to mask the illumination gaps formed about the peripheral of the transparency proper. The frame can also be used to make notes or other memoranda needed by the lecturer which remain unseen by the audience. However, these frames are bulky and will not fit into standard size files, binders and shelves, being therefore difficult to store and transport. Also, they are not easily removable without destruction of the frame and attendant damage to the transparency. Finally, the frames are not convenient to attach to transparencies as each one must be carefully positioned and attached by hand.

Swedish Patent Application No. 382,266 discloses an envelope which is a plastic sheet pocket having punched holes along one side, and being open at the top for insertion of the transparency to be presented. These envelopes are removable, simple to handle, store and transport as the outer dimensions are those of standard binders or files. Moreover, the transparency is protected by the envelope and at the presentation, notes can be made on the envelope.

The disadvantage of this envelope is that peripheral illumination is not masked and the lecturer cannot make written notes on the envelopes without having these notes being shown on the screen as well. The punched holes also show an outline on the screen. Therefore, this envelope does not yield a presentation having an extremely professional appearance.

U.S. Pat. Nos. 3,253,358; 3,264,936; 3,438,702; 3,438,703; 3,524,703; 3,536,393; 3,537792; 3,544,211; 3,600,079; and Swedish Patent Application No. 346,166 describe further examples of covers or envelopes for OHP transparencies. The covers and envelopes disclosed in these publications are complicated and difficult to handle and have many drawbacks, e.g., most of these presuppose that the transparencies are to be presented in a prescribed order and the covers of most of these transparencies are difficult to remove.

U.S. Pat. No. 4,810,544 discloses another transparent sleeve comprising a transparent sheet material having a single fold dividing the sheet into two halves, and a strip of double coated adhesive tape running the length of an edge of the sheet substantially parallel to the fold. When folded, the single sheet forms a sleeve. This envelope protects the printed media, and can be stored in a binder when holes are punched along the edge, but again, any written notes or prompts will be visible on the screen.

U.S. Pat. No. 4,402,585 discloses an envelope for overhead transparencies that overcomes many of the disadvantages of the previously described envelopes. The envelope comprises a rectangular pocket formed of transparent sheet material defining opposed rectangular faces which are separable along at least one side edge for insertion of the transparency therebetween. At least one opaque flap is attached in a foldable manner along one of the longitudinal side edges of the pocket. The flap is attached to and spaced a distance from the longitudinal edge such that in the unfolded position the flap extends beyond the side edge and covers the longitudinal edge. In the folded position, the flap exposes the longitudinal side edge. Holes may be punched along the side edge of the material to permit storage of the envelope in a binder, file or the like. Preferably, the flaps are formed of a plastic material which accepts written text by conventional writing instruments.

When stored, the flaps are in the folded position, and are unfolded for use. The means of attachment must allow the flaps to stay in the unfolded position during presentation. Otherwise the flaps might tend to spring back to their folded position, which is inconvenient for the presenter, and may cover the added notes. To aid the situation, the envelope can be placed with the flaps face down on the overhead projector stage, so that the weight of the envelope and the overhead transparency can insure that the flaps stay unfolded. However, where flaps are difficult to fold, the envelope will not lay flat on the stage, which can cause focusing problems, which are especially severe when one uses a reflective type overhead projector. In addition, due to the operating temperature of the OHP and stage, the adhesive used to attach the flap can be transferred to the stage, causing unsightly spots on the screen and messy blotches on the stage which can also transfer to materials used thereafter.

The present inventors have discovered that, by using a certain combination of backing film and adhesive, the flaps can be securely attached to the transparent envelope, easily remaining in either the folded or the unfolded or open position. This obviates the need to turn the envelopes face down on the stage. Further, with the preferred composite tapes, the envelopes can be turned face down and lay flat on the stage with minimal adhesive transfer.

SUMMARY OF THE INVENTION

The present invention comprises an improved envelope for transparencies for overhead projectors, said envelope comprising a substantially rectangular pocket of transparent sheet material, said pocket having two longitudinal side edges connected by shorter side edges, said pocket being open at least along one side edge for insertion of a transparency, at least one opaque flap attached in a foldable manner along one of said side edges by means of a flexible adhesive tape, said flap having a folded and an unfolded position, said tape comprising a flexible polyolefin backing having a thickness of about 30 micrometers or less, and attached on one surface thereof, an adhesive layer, wherein said tape has a flexibility measurement of not more than about 2.5 grams, and a 180° peel adhesion of at least about 100 grams per centimeter, wherein the flap, when unfolded, remains in the unfolded position, and when turned face down on an overhead projector stage will exhibit minimal adhesive transfer to said stage at the operating temperature of the overhead projector.

This invention also provides an adhesive tape suitable for use in an improved envelope for overhead transparencies, having at least one opaque flap, such tape comprising a flexible polyolefin material having a thickness of less than about 30 microns, and coated on one surface of said backing is a flexible adhesive layer wherein said composite tape has a flexibility measurement of not more than 2.5 grams, and a 180° peel adhesion of at least about 100 grams per centimeter, wherein said tape will exhibit minimal transfer to an overhead projector stage at the operating temperature of the overhead projector.

DESCRIPTION OF THE DRAWINGS

In FIGS. 1 and 2, the envelope 1 comprises a pocket 2, which consists of two sheets 4, 5 of transparent plastic material, which can hold a transparency 3, said sheets being joined together at least along one side or formed by folding a large sheet to form the faces of the envelope. Along one longitudinal side, the envelope may be provided with punched holes 6 to permit storage in a file, binder or the like. Two opaque, substantially rectangular elongated flaps 7, 8 are attached in a foldable manner to the outer side of one sheet 5 of the pocket or envelope by means of an adhesive tape 9.

DETAILED DESCRIPTION OF THE INVENTION

The improved envelopes of the invention comprise an adhesive tape having a polyolefin backing having a thickness of 30 micrometers or less, preferably from about 10 micrometers to about 25 micrometers. The backing may be any thermoplastic film forming polyolefin, including but not limited to, polyethylene, especially low density polyethylene and linear low density polyethylene, polypropylene, polyalkyl-α-olefins, and polyallomers, e.g., copolymers or blends of ethylene, propylene and copolymers having a major portion of 3-methyl-1-butene, and a minor part of a straight chain n-alkene having from 2 to 18 carbon atoms such as 1-octene, 1-hexadecene and the like. Crosslinked polyolefins tend to have less flexibility than those which are not crosslinked. Therefore, when crosslinked polyolefins are used, thinner backings will be required. Preferred polyolefins include polypropylene and linear low density polyethylene.

Certain polyethylene blends tend to show more adhesive transfer when tested because they have less integrity and tend to stretch to the breaking point when dragged across the overhead projector stage. This stretching allows more adhesive to be exposed, and ultimately, to transfer. When an aggressive adhesive is used in combination with these blends, the backing will be stretched to the breaking point in testing. In actual use however, such high stresses are not generally placed on the pocket, and such backings may give perfectly acceptable amounts of transfer.

Polyolefin films are commercially available from many companies, such as DuPont and Consolidated Thermoplastics Company.

Useful adhesives for the adhesive layer of such tape are those having good adhesion to the polyolefin backing, and the envelope, and include acrylics, tackified or untackified, tackified rubber-based adhesives, urethanes, silicones and the like. They can be hot melt adhesives, solvent-based, water-based, or emulsion, E-beam or actinic radiation curable or moisture curable.

The thickness of the adhesive layer depends on such factors as the type of adhesive used and the type and thickness of the film backing, however, the thicknesses of the adhesive layer range from about 5 micrometers to about 150 micrometers, preferably from about 10 micrometers to about 75 micrometers, most preferably from about 10 to about 25 micrometers.

At this thickness range, the adhesive layer has a 180° peel adhesion of at least 100 grams per centimeter (under conditions specified, infra), for the flaps to remain securely attached to the edges of the pocket. When adhesives with lower peel adhesion values are used, the adhesive will tend to separate from the edges of either the main pocket or the flap after repeated opening and closing of the flaps.

The adhesives can be pressure-sensitive, heat activatable pressure-sensitive, or heat bonding with the pressure-sensitive and heat-activatable pressure-sensitive being preferred. Also useful are heat-set adhesives, which have the advantages of good bonding strength to prevent separation of the adhesive, and are nontacky at the operating temperature of the overhead projector, so long as they meet the flexibility and peel requirements necessary. Useful pressure-sensitive adhesives possess good bonding and low creep properties, while remaining flexible, which is one requirement so that when the flap is in the unfolded position, there will be less tendency for the adhesive to creep onto the stage or other surface contacted. The thin backing of the tape and resultant flexibility provides a smaller exposed area when the flap is in the unfolded position. Minimal adhesive layer thickness also helps in minimizing amount of adhesive which is exposed when the flap is in the unfolded position. These factors, along with the adhesive properties substantially reduce the amount of adhesive transfer when compared to conventional products.

Other helpful procedures include detackifying the exposed adhesive edge along the fold line, coating variations and the like. Useful detackifying treatments include treating the exposed edges with E-beam or the addition of an appropriate chemical compound to either coat or react with the adhesive.

Preferred adhesives include acrylics and tackified rubber-based adhesives. For the preferred adhesives, a very thin layer of about 10 micrometers can provide the required peel adhesion.

One preferred class of pressure-sensitive adhesives are acrylate adhesives. They can be monomers and/or oligomers such as (meth)acrylates, (meth)acrylamides, vinyl pyrrolidone and azlactones, as disclosed in U.S. Pat. No. 4,304,705, (Heilmann), Re 24,906 (Ulrich), and U.S. Pat. No. 4,181,752, (Martens), all or which are incorporated herein by reference. Such monomers include mono-, di-, or poly-acrylates and methacrylates.

Preferred acrylates are typically alkyl acrylates, preferably monofunctional unsaturated acrylate esters of non-tertiary alkyl alcohols, the alkyl groups of which have from 1 to about 14 carbon atoms. Included with this class of monomers are, for example, isooctyl acrylate, isononyl acrylate, 2-ethyl-hexyl acrylate, decyl acrylate, dodecyl acrylate, n-butyl acrylate, and hexyl acrylate. The alkyl acrylate monomers can be used to form homopolymers or they can be copolymerized with polar copolymerizable monomers. When copolymerized with strongly polar monomers such as monoolefinic mono- and dicarboxylic acids, hydroxyalkyl acrylates, cyanoalkyl acrylates, acrylamides or substituted acrylamides, the alkyl acrylate monomer generally comprises at least about 75% of the polymerizable monomer composition. When copolymerized with moderately polar monomers such as N-vinyl pyrrolidone, acrylonitrile, vinyl chloride or diallyl phthalate, the alkyl acrylate monomer generally comprises at least about 70% of the polymerizable monomer composition.

The pressure-sensitive adhesive typically contains at least one initiator to aid in polymerization of the monomers. Suitable initiators include thermally-activated initiators such as azo compounds, hydroperoxides, peroxides, and the like, and photoinitiators such as the benzoin ethers, substituted benzoin ethers, substituted acetophenones, substituted alpha-ketols such as 2-methyl-2-hydroxy-propiophenone, aromatic sulphonyl chlorides such as 2-naphthalene sulphonyl chloride, and photoactive oximes such as 1-phenyl-1,1-propanedione-2-(O-ethoxycarbonyl) oxime. Generally, the initiator is present in an amount of from about 0.01 part to about 3.0 parts based on 100 parts monomer weight.

The pressure-sensitive adhesive layer may also be cross-linked. Preferred crosslinking agents for acrylic pressure-sensitive adhesives are multiacrylates such as 1,6-hexanediol diacrylate as well as those disclosed in U.S. Pat. No. 4,379,201 (Heilmann et al.), incorporated herein by reference. Other useful crosslinking agents include those taught in U.S. Pat. Nos. 4,330,590 (Vesley), and 4,329,384 (Vesley et al.), both of which are also incorporated by reference. Each of the crosslinking agents is useful in the range of from about 0.01% to about 1% of the total weight of the monomers.

Useful heat activatable adhesives include thermoplastic and thermosetting resins. Another preferred class of adhesives is that of rubber-type adhesives. The adhesives comprise a rubber resin, which may be either a natural rubber or a synthetic elastomer such as styrene-butadiene and styrene-butadiene-styrene copolymers and hydrogenated copolymers thereof, polybutadiene, polyisoprene, polyisobutylene, butyl rubbers, polychloroprene, ethylene-propylene copolymer rubbers, ethylene-propene-diene rubbers, and the like. Mixtures of such rubber resins may also be used. Such resins are available commercially under such trade names as Kraton™ e g., Kraton™ G1657, D1101, D1107, and 1118, all available from Shell Chemical Co., Stereon™, e.g., Stereon™ 840A, and 845A, and Duradene™ e.g., Duraden™ 710, 711, and 713, all available from Firestone Tire and Rubber, those available from BF Goodrich under the tradename Hycar™, and the like. Such rubber-type adhesives typically also contain a tackifying agent. Useful tackifying agents include rosin esters and rosin acids, mixed aliphatic/aromatic tackifying resins, polyterpene tackifying agents, and hydrogenated tackifying agents. Preferred tackifying agents include the aliphatic hydrocarbon resins, the polyterpene resins and the hydrogenated resins. Mixtures of tackifying agents may also be used.

Rubber-type adhesives may also contain a plasticizing agent including naphthionic oils, paraffinic oils, aromatic oils and mineral oils.

Useful commercially available adhesives include Findley "590-371", and "802-334"; "HL-2203-X", "HM-2703", "HL-7389-X", available from H. B. Fuller, and "70-7141", useful adhesive may also contain such conventional additives as pigments, antioxidants, fillers, and the like, so long as such additives do not render the adhesive outside the flexibility and peel ranges required.

The adhesive chosen must have good bonding to the film backing, either as provided or the surface of the film backing can alternately be treated either by corona treatment or chemical adhesion promoters to insure good bonding. The composite tape has a flexibility measurement of no more than about 2.5 grams as measured infra, preferably no more than about 1.6 grams. Preferred adhesives also have a 180° peel adhesion of at least about 100 grams per centimeter, preferably at least about 150 grams per centimeter; and a composite thickness of less than about 150 micrometers, preferably less than about 95 micrometers.

The flexibility measurement requirement can be as low as possible, but in practice, measurements of lower than about 1 gram become difficult to test. A tape with very low flexibility numbers, or outside the testing range is useful if it has adequate peel, and adequate integrity needed for the equipment processing and handling. At measurements higher than about 2.5 grams, the flaps are difficult to unfold and will tend not to remain in the opened or unfolded position and to spring back to the closed position.

The backing can also have low adhesion backsize coating thereon or any other treatment to facilitate unwinding from a roll, so long as such treatments do not render the tape outside the flexibility range.

The flaps are preferably made from materials that are capable of being overwritten to enable the lecturer to make notes thereon, including bond paper, paperboard, opaque films such as pigment filled polyester; polyvinylchloride; polyolefins including polyethylene and polypropylene; and other similar materials, with polyvinyl chloride being preferred.

The pockets can be made from any transparent polymeric sheet material including cellulose esters, polyesters such as polyethylene terephthalate, polyamides such as nylon, polyolefins such as polyethylene and polypropylene, polycarbonates, polyethers and polyvinyls such as polyvinylchloride. Polyesters are preferred, most preferably polyethylene terephthalate.

The pockets comprise opposed rectangular faces which are separable at least along one side edge for insertion of the transparency therebetween. The phrase "along one side edge" includes embodiments where the flap is attached along and coextensive with the longitudinal side edge of the pocket, as well as those where they are spaced a distance from the same side edge as disclosed in U.S. Pat. No. 4,402,585, incorporated herein by reference, such that in the unfolded position the flap extends beyond and covers the longitudinal edge. In the folded position the flap exposes the side edge. Where desired, the flap can also be attached along the short side edge.

Holes may be punched along the side edge of the pocket to permit storage of these pockets in a binder, file or the like.

TEST METHODS Flexibility Measurement

The flexibility of the adhesive tape is measured with a Sherr-Tumico tension gauge having a range of measurement from 0.6 to 6.0 grams of tensions. A driven platen is used to maintain a constant and reproducible speed of about 30.5 cm/minute (12 inches/minute). Samples are measured using white polyvinyl chloride flaps having dimensions of about 4.5 cm ×29 cm and thickness of 120 micrometers. The flaps weigh about 2.1 grams. The flap is attached to a pocket using the tape to be tested. The sample is placed, flap side up, on a flat surface with the flap opened at a 90° angle from the sleeve. The sample is set so that when the platen starts moving, the tension gauge arm will engage the flap by approximately 0.65 cm. (The flip extends 0.65 cm into the path of the arm,), and at a position about 2.54 cm above the flap/pocket intersection. The tension gauge is mounted on the driven platen so that it is moving at 30.5 cm per minute when it engages the flap. The platen is started and the tension gauge moves horizontally until it engages the flap and forces it to open further. The maximum tension is read from the gauge after it clears the flap. The result is reported in grams. Results are comparable for flaps having a weight specified above. If testing is desired for flaps having other weights, the results should be specified in grams per unit weight or unit area.

180° Peel

Peel adhesion is measured using an SP-102B-3M90 Extended Capability Slip/Peel tester, marketed by IMASS, Inc. The instrument is set up as follows: platen speed of 30.5 cm/minute, averaging time selector set to 10 seconds, peak select set to kinetic, meter select switch to peak, speed range selector to SL, and range/zero switch to B. A sample is cut parallel to the tape so that the flap, tape and pocket are intact when testing a commercially available example. The tape is applied directly to either the flap or the pocket alone for all other samples. Results are specified as being peel from PET if from the pocket, PVC for peel from the flap or both.

To measure peel adhesion of the hinge to the pocket, the sample is secured to the platen with double coated tape and the flap is peeled from the pocket to a distance of about 2.54 cm. The flap is attached to the load cell on the peel tester and the platen is started. As the platen moves, the flap is peeled from the pocket and the force is measured by the load cell and displayed in grams. Peel adhesion to the flap is measured by peeling the tape from the flap. Peel adhesion is reported in grams/cm, and represents the average of 4 measurements.

Adhesive Transfer Test

A 3M Model 2150 Overhead Projector was used in observe adhesive transfer in the following manner:

The overhead projector was turned on and allowed to warm for 30 minutes. A 4 cm sample, cut perpendicular to the hinge, so that the hinge, pocket and flap interaction was intact, was placed on the stage, face down, with the flap in the open position. A cylindrical brass weight having a diameter of 3-8 cm and weighing 100 grams was placed and centered on open hinge and let stand for about 1 minute. The sample was then pulled across the stage at a rate of 30.5 cm per minute (12 inches per minute) in such a direction that the flap was forced to open further to expose the adhesive. Three samples were tested unless otherwise noted. The result was reported as P/3 if no adhesive residue was left on stage, if adhesive residue was left on stage, the result was reported as F/3. The sample is deemed to be acceptable if two of three samples do not leave residue.

The following examples are meant to be illustrative only. Variations within the spirit of the invention are possible as understood by those skilled in the art. The scope of the invention is that contained in the claims.

EXAMPLES Example 1

A pocket for overhead transparencies is made in the following manner. A hot melt pressure-sensitive adhesive, available as Findley 590-371 is coated by means of a Nordson Slot Nozzle Coater at a thickness of about 71 micrometers onto a 125 micrometer polyethylene-coated release liner. The adhesive was then transferred to a 19 micrometer thick low density polyethylene film (LDPE) with white pigmentation, available as LSF-40 W884M from Consolidated Thermoplastics Co.

The tape is then applied to a transparent PET pocket. The flaps, made of TiO2 filled PVC film having dimensions of about 4.5 cm ×29 cm and thickness of 120 micrometers are attached to the pocket using the above tape. The finished pocket is then tested for 180° peel adhesion, adhesive transfer, and flexibility according to the methods described above and the test results are reported in Table 1.

Example 2

This envelope was made in the same manner as Example 1, except that the adhesive layer was an acrylate adhesive comprising 95.5 parts isooctyl acrylate (IOA) and 4.5 parts acrylic acid (AA), coated out of solvent (27% solids in ethyl acetate) at a dry thickness of 12.5 micrometers which was coated using a knife coater onto an identical 19 micrometer LDPE film. The tape was then used to attach the flaps to the edge of the pocket. The tape was tested for peel, flexibility and adhesive transfer. The measurements are also reported in Table 1.

Examples 3 and 4

These samples were made in the same manner as Examples 1 and 2, respectively, except that different adhesive thicknesses were used, as shown in Table 1. The peel, adhesive transfer and flexibility measurements were made, and the results are shown in Table 1.

Examples 5-9 and Comparative Examples 10C-12C

These envelope were made in the same manner as Example 2, except that the adhesive was an acrylate adhesive comprising 90 parts 2-methylbutyl acrylate, and 10 parts AA in ethyl acetate at 40% solids, and were coated using a knife coater to different backings at varying thicknesses. These are listed in Table 2. The peel, adhesive transfer and flexibility data are included in Table 1.

Example 13C

This example was a commercially available Flip-Frame™ product, available from 3M, with an acrylate adhesive comprising 95.5 parts IOA and 4.5 parts AA, coated out of solvent (27% solids in a 70/30 mixture of heptane/isopropanol) at a coating weight of 32 g/m². The backing for the tape was a 130 μm thick rayon non-woven sheet. The envelope was tested for peel, adhesive transfer and flexibility, and the test results are also reported in Table 1.

Examples 14-17

These samples were made in the same manner as Example 2 except that different adhesive thicknesses were used. These samples were also tested for peel, adhesive transfer and flexibility and the results reported in Table 2.

Examples 18-19

These samples were made in the same manner as Example 2 except that the adhesives used Were PR#9471, and PR#467MP, commercially available Scotch™ acrylic transfer adhesives from 3M. These samples were also tested for peel, adhesive transfer and flexibility and the results reported in Table 2.

Examples 20-21

These samples were made in the same manner as Example 1 except that the backings were soft polypropylene, available as XBPP-14.0 from Consolidated Thermoplastics Company, used in varying thicknesses, and that the adhesive used was a 90/10 ratio of 2-methylbutyl acrylate. The compositions are reported in Table 2. These samples were also tested for peel, adhesive transfer and flexibility and the results reported in Table 2.

Example 22C

This sample was made in the same manner as Example 20, except that the adhesive coating was 100 micrometers. As can be seen from the data in Table 2, this sample, which is outside the limits of the invention, failed the flexibility test.

                                      TABLE 1                                      __________________________________________________________________________          FLEXI-                                                                              BACKING                                                                              BACKING ADHESIVE                                                                               180° PEEL                               EX-  BILITY                                                                              COMPO-                                                                               THICKNESS                                                                              THICKNESS                                                                              (g/125 mm)                                                                             ADHESIVE                               AMPLE                                                                               (g)  SITION                                                                               (μm) (μm) PVC PET TRANSFER                               __________________________________________________________________________     1    0.6  LPDE  19.0    71.0    >638.sup.1                                                                         >440.sup.1                                                                         2P/3                                   2    1.0  LPDE  19.0    3.0      162                                                                                374                                                                               3P/3                                   3    0.8  LPDE  19.0    132     >500.sup.1                                                                         >440.sup.1                                                                         3P/3                                   4    0.7  LPDE   9.0    33.5    >400.sup.1                                                                         >200.sup.1                                                                         2P/3                                   5    1.0  PE.sup.2                                                                             19.0    2.5     >460.sup.1                                                                          --.sup.1                                                                          2P/3                                   6    0.6  LLS-19.sup.3                                                                         12.2    4.1     >400.sup.1                                                                          317                                                                               3P/3                                   7    0.8  SS-11.sup.3                                                                          19.0    3.2      505                                                                                400                                                                               3F/3                                   8    1.5  LLS-19.sup.3                                                                         31.8    19.0     432                                                                                284                                                                               3F/3                                   9    0.6  LSF-40.sup.4                                                                         12.2    50      >300.sup.1                                                                         >300.sup.1                                                                         2P/3                                   10-C 4.8  LMAX.sup.5                                                                           50      30       352                                                                                315                                                                               2F/3                                   11-C 1.1  Cryovac                                                                              19.0    46       413                                                                                300                                                                               2F/3                                             D955                                                                 12-C 2.4  Cryovac                                                                              30.2    22       234                                                                                245                                                                               3F/3                                             D955                                                                 13-C 4.1  Nonwoven                                                                             130     --       712                                                                                218                                                                               5F/5                                   __________________________________________________________________________      .sup.1 Film broke                                                              .sup.2 940 LSF40 488 4M, available from Consolidated Thermoplastics            Company                                                                        .sup.3 Linear low density polyethylene/LDPE(LLDPE) blend available from        Consolidated Thermoplastics Company                                            .sup.4 LDPE, available from Consolidated Thermoplastics Company                .sup.5 100% octane based (C)PE, available from Consolidated Thermoplastic      Company                                                                        .sup.6 Multilayered, crosslinked polyolefin, available from WR Grace &         Company, Cryovac Division                                                

                                      TABLE 2                                      __________________________________________________________________________                     FLEXI-                                                                              BACKING ADHESIVE                                                                               180° PEEL                          EX-  ADHESIVE   BILITY                                                                              THICKNESS                                                                              THICKNESS                                                                              (g/.125 mm)                                                                            ADHESIVE                          AMPLE                                                                               FORMULATION                                                                               (g)  (μm) (μm) PVC PET TRANSFER                          __________________________________________________________________________     14   IOA/AA 26% 0.6  19.0.sup.1                                                                             18      >500.sup.3                                                                         >475.sup.3                                                                         1P/3                                   (98/2) solid in                                                                ethylacetate/heptane                                                      15   IOA/AA 29% 1.0  19.0.sup.1                                                                             30      >500.sup.3                                                                         >475.sup.3                                                                         3P/3                                   (90/10) solid in                                                               ethylacetate/toluene                                                      16   IOA/OA/SSS 1.1  19.0.sup.1                                                                             30       297                                                                                292                                                                               2P/3                                   (87/12/1)                                                                 17   IOA/ACM (94/6)                                                                            0.8  19.0.sup.1                                                                             15      >336.sup.3                                                                         >716.sup.3                                                                         2P/3                                   21% solid in                                                                   ethylacetate/heptane                                                      18   PR#9471 Lot 1022                                                                          0.6  19.0.sup.1                                                                             65      >593.sup.3                                                                         >580.sup.3                                                                         3F/3                              19   PR#467MP Lot 1087                                                                         1.0  19.0.sup.1                                                                             32       406                                                                               >560.sup.3                                                                         3P/3                              20   Findley 590-371                                                                           0.6  12.5.sup.2                                                                             28      >390.sup.3                                                                         >365.sup.3                                                                         2P/3                              21   Findley 590-371                                                                           1.2  30.sup.2                                                                               29       --  -- 2F/3                              22-C Findley 590-371                                                                           4.0  62.sup.2                                                                               30       318                                                                                234                                                                               2F/3                              __________________________________________________________________________      .sup.1 Low density polyethylene, "LSF40" W884M, available from                 Consolidated Thermoplastics Company                                            .sup.2 Polypropylene, XVPP"14.0" from Consolidated Thermoplastics Company      .sup.3 Backing breaks                                                     

What is claimed is:
 1. An improved envelope for transparencies for overhead projectors, said envelope comprising a substantially rectangular pocket of transparent sheet material, said pocket having two longitudinal side edges connected by shorter side edges, said pocket being open at least along one side edge for insertion of a transparency, at least one opaque flap attached in a foldable manner along one of said side edges by means of a flexible adhesive tape, said flap having a folded and an unfolded position, said tape comprising a flexible polyolefin backing having a thickness of less than about 30 micrometers, and attached on one surface thereof, an adhesive layer, wherein said tape has a flexibility measurement of not more than about 2.5 grams, and a 180° peel adhesion of at least about 100 grams per centimeter, said flap being capable of remaining in the unfolded position on an overhead projector stage with minimal adhesive transfer to said stage at the operating temperature of the overhead projector.
 2. An improved envelope according to claim 1 wherein said tape has a flexibility measurement of no more than 1.6 grams.
 3. An improved envelope according to claim 1 wherein said tape has a 180° Peel adhesion of at least about 150 grams per centimeter.
 4. An improved envelope according to claim 1 wherein said polyolefin backing is selected from the group consisting of polyethylene, linear low density polyethylene, polypropylene, polyalkyl-α-olefins, and polyallomers.
 5. An improved envelope according to claim 1 wherein said polyolefin backing has a thickness of from about 10 micrometers to about 25 micrometers.
 6. An improved envelope according to claim 1 wherein said adhesive a rubber resin adhesive selected from the group consisting of natural rubber, styrene-butadiene and styrene-butadiene-styrene copolymers and hydrogenated copolymers thereof, polybutadiene, polyisoprene, polyisobutylene, butyl rubbers, polychloroprene, ethylene-propylene copolymer rubbers, ethylene-propene-diene rubbers, and mixtures thereof.
 7. An improved envelope according to claim 4 wherein said adhesive is a tackified styrene-butadiene-styrene copolymer.
 8. An improved envelope according to claim 1 wherein said adhesive is an acrylic adhesive.
 9. An improved envelope according to claim 8 wherein said adhesive comprises isooctyl acrylate and acrylic acid. 